There are long lists of weights & scores & reasons emails are dumped into the junk box. Unfortunately, that information isn't published. If it was, any spammer could go through the rules & craft emails that would slip through the cracks fairly easily.
If you're trying to keep emails in your network from entering users' junk boxes, you can setup "safe-sender" lists & deploy them via GPO...
If you are sending emails to another office & they're incorrectly being marked as junk... you're going to have a hard time overcoming this. Generally, you need to look at your email, and verify all the details appear accurate (doubly-so in the message headers) Look for things like the sent date & time are not unrealistic... check on common spam sender lists to see if your domain/ip is not blacklisted... http://www.mxtoolbox.com/blacklists.aspx is a good tool for that.
Also avoid common spam phrases like "advertisement" and "viagra" in the subject lines. Email addresses with numbers in the first 8 characters are also a common red-flag. There's a few lists out there with more info, but here's a pretty short one: http://www.streamsend.com/kb2/idx.php/13/106/05-Delivery/article/Tips-to-Avoid-the-Outlook-Junk-Folder.html
